People often have to get the test, called a PPD test, before enrolling in school, and it's also often required annually for people who work in hospitals, nursing homes, jails and other facilities where exposure to tuberculosis can be a problem.

"We believe we've identified the cause and we're back on track, but it will take a while to catch up," said Len Lavenda, a spokesman for Sanofi Pasteur Limited, manufacturer of the Tubersol PPD test. He told Shots he didn't "know specifically" what led to the production halt at the Toronto plant.

The Tubersol PPD test will be largely unavailable until the end of May, according to the Centers for Disease Control and Prevention, which just published a report on the shortage in Morbidity and Mortality Weekly Report.

There's just one other PPD test approved in the United States, and that one, Aplisol, has been increasingly hard to find as health-care providers turn to it because they can't get Tubersol. As a result, the CDC reports, "This might require deferment of testing some persons."

PPD tests work by injecting a fluid containing sterile bits of protein under the skin on a person's forearm. If the person has been infected with the tuberculosis bacillus, the test can cause a raised, hard welt on the arm in two to three days. (MedlinePlus has a good description of the test, and the CDC goes into more detail on who should get tested.)

TB skin tests are cheap ($39 at Minute Clinic) and simple, though they do involve a needle and a trip back to the doctor if the test looks positive. A positive test doesn't mean a person has an active TB infection, but does need followup.

If PPD tests can't be found, the CDC recommends an IGRA blood test. It's a bit more invasive because it requires a blood draw, but can be more accurate, particularly in people who have had a BCD TB test, which is used in other countries.